湖南浏阳永和碳酸盐岩中的天青石

湖南浏阳永和天青石赋存于二叠系栖霞组,由生物屑泥灰岩或泥晶灰岩构成的灰岩瘤状体中。瘤状体中含一团或数团呈放射状类似菊花的天青石晶簇。 分析结果证实,本区天青石晶体成分纯净,仅含有少量方解石。据化学分析数据,运用氧原子法计算,其晶体化学式是（Sr 0.97600,Ba 0.0012）SO₄。 野外和镜下观察以及碳、氧同位素分析表明,菊花状天青石是在碳酸盐沉淀后,在成岩阶段,由于介质性质（Eh、pH等）的变化,原先呈分散状分布的天青石重新溶解,向弱酸性和氧逸度较高的地方聚集结晶而成。

CELESTITES OF CARBONATE IN YONGHE,LIUYANG COUNTY,HUNAN PROVINCE

The celestites ( SrSO₄) , found in Yonghe, Liuyang County, Hunan Province, wich occur in carbonates of the Chihsia formation of the Lower Permian, are associated with sepiolite deposites. Since the celestite is an important industrial mineral, the discovery of it in this area is of some economic value. The single crystals of the celestite in Yonghe are long-column. Commonly surrounds the siliceous detritus and shows druses texture as chrysanthemum shape.Therefore,it is called "chrysanthemum-stone" too. By the observation of microscope, the refractive index and the other optical properties of the celestite are as follows: Ng = 1.630, Np = 1.622, positive biaxial crystal and middle ( 2V ) angle. Its X-ray diffraction pattern is characterised by peaks, d = 2.731 A , d = 3.972 A , d = 3.295A.There is another characteristic peak of the calcite,d = 3.03 A .In the infrared absorption spectrum pattern there is spectrum of the standard celestite such as 1295.16cm-1, 1195.56cm-1, 1136.81cm-1, 1090.76cm-1, 991.81cm-1, 640.74cm-1, and 610.64cm-1.The differential thermal analysis ( DTA ) curve shows two endothermic peaks. One is relateed to 740°C, perhaps this results from a small amount of the carbonate mineral in the sample. Another appears at 1146°C and its shape is sharp and wide.This is the characteristic peak of celestite created by the isodimorphism.From the analysis results mentioned above, the authors consider that the mineral composition of the celestite deposited in Yonghe is mainly celestites and a few calcites. The chemical composition of the celestite from Yonghe is as follows: SrO 54.21%,CaO 1.65%,MgO 0.25%,BaO 0.01%,A12O3 0.04%, Fe 2O3 0.02% and SO343.32% According to the calculation of oxygenatom, its crystal-chemcal formula is (Sr 0.97600, Ba0.0012)0.97612SO4. The origin of the celestite in Yonghe is very interesting.The analysis of Sr-iso-tope indicated that the strontium of the celestite was derived from sea water.It can be suggested that the chrysanthemum-shape celestite formed the during diagenetic period, after carbonates precipitated, according to the characteristics; ( 1 ) The chrysanthemum-shape celestites grew around fossil debris of brachiopoda and gastropoda. ( 2 ) The celestites are centred in the nodular limestones which formed during diagenetic period. ( 3 ) The celestites replaced the early calcites. ( 4 ) The carbonate suffered strong diagenesis based on the analysis results of carbon and oxygen isotope of calcite, δ13C = 3.720,δ18O=-4.269.